The Mars Surveyor '01 Rover and Robotic Arm

Robert G. Bonitz, Tam T. Nguyen, Won S. Kim Jet Propulsion Laboratory California Institute of Technology 4800 Oak Grove Drive Pasadena, CA 91109-8099 818-354-4628 bonitz @ telerobotics.jpl, nasa. gov

Abstract - The Mars Surveyor 2001 Lander will carry with Robotic Arm system and operations are described in section it both a Robotic Arm and Rover to support various science 2. and technology experiments. The Marie Curie Rover, the twin sister to Sojourner Truth, is expected to explore the surface of Mars in early 2002. Scientific investigations to determine the elemental composition of surface rocks and soil using the Alpha Proton X-Ray Spectrometer (APXS) will be conducted along with several technology experiments including the Mars Experiment on Electrostatic Charging (MEEC) and the Wheel Abrasion Experiment (WAE). The Rover will follow uplinked operational sequences each day, but will be capable of autonomous reactions to the unpredictable features of the Martian environment.

The Mars Surveyor 2001 Robotic Arm will perform rover deployment, and support various positioning, digging, and sample acquiring functions for MECA (Mars Environmental Compatibility Assessment) and Mossbauer Spectrometer experiments. The Robotic Arm will also collect its own sensor data for engineering data analysis. Figure 1 Mars Surveyor 2001 Lander The Robotic Arm Camera (RAC) mounted on the forearm of the Robotic Arm will capture various images with a wide After the Robotic Arm deploys the Marie Curie rover, the range of focal length adjustment during scientific sister of the Mars Pathfinder Sojourner rover, onto the experiments and rover deployment Martian surface, the rover will begin traversing the surface in the vicinity of the Lander. The rover visiting locations 1. INTRODUCTION will be designated by a human operator using engineering data collected during previous traversals and end-of-sol The Mars 2001 Surveyor Lander is the next mission in the (Martian day) stereo images captured by the Lander stereo Mars Surveyor Program whose primary objective is to cameras [6]. During the traversals the rover will further our understanding of the biological potential and autonomously avoid rock, drop-off, and slope hazards. It possible biological history of Mars, and to search for will change its course to avoid these hazards and will turn indicators of past and/or present life. The Lander (Figure 1) back toward its goals whenever the hazards are no longer in is scheduled to land on the equatorial region (3N to 12S) of its way. The rover uses "dead reckoning" counting wheel Mars on Jan. 27, 2002. It is a platform for science turns and on-board rate sensors to estimate position. instruments and technology experiments designed to Although the rover telemetry will record its responses to provide key insights to decision regarding successful and human driver commands in detail, the vehicle's actual cost-effective human missions to Mars. Two key positions will not be known until examination of the instruments are the Robotic Arm and the Marie Curie Lander stereo images at the end of the sol. The rover will Rover. stop at several sites of interest for various scientific and engineering experiments. The primary purpose of the Robotic Arm is to support the other science instruments by digging trenches in the 2. ROBOTIC ARM Martian soil, acquiring soil samples, positioning arm- mounted science instruments near or on appropriate targets, The Mars Surveyor 2001 Robotic Arm (Figure 2) is a low- and deploying the Marie Curie Rover to the surface. It will mass 4-degree-of freedom manipulator with a back-hoe also be used to conduct soil mechanics experiments to design [91 inherited ti'om the Mars Surveyor '98 Robotic investigate the physical properties of the surface and Arm. The end effector (Figure 2) consists of a scoop for subsurface materials in the workspace. Details of the diggingandsoilsampleacquisition,secondarybladesfor soilsampleacquisition,thescoopwill bepositionedforthe scraping,anelectrometerformeasuringtriboelectriccharge RACto takeclose-upimagesof thesoilsamplesin the andatmosphericionization,andacrowfootfordeploying scooppriortodeliverytotheMECA.Thereisadivotin the theRoverfromtheLanderto thesurface.Controlof the scoopbladeto containsmallsoil samplestbr veryclose Armisachievedbyacombinationofsoftwareexecutingon imagingbytheRACatadistanceof 1imm.TheArmwill theLandercomputerandfirmwareresidentin theRobotic alsopositionthe RACfor imagingof the patchplate Arm electronics.The RoboticArm is an essential locatedontheMECA,nearbyrocks,andanyotherobjects instrumentin achievingthescientificgoalsof theMars ofscientificinterestwithinitsworkspace. Surveyor2001missionbyprovidingsupportto theother Mars Surveyor2001scienceinstrumentsas well as Support to the Mossbauer Spectrometer - The Mossbauer conductingArm-specificsoilmechanicsexperiments. Spectrometer is located on the Robotic Arm forearm between the elbow and RAC and is used to determine the composition and abundance of iron-bearing minerals. The Robotic Arm will position the Mossbauer on its calibration and magnetic targets located on the Lander deck as well as on soil targets within the reach and kinematic constraints of the Arm.

Support to the Marie Curie Rover - In the historical 1997 Pathfinder Mission, a ramp pathway was used to drive the Sojourner Truth Rover from the Lander deck to the Martian surface. In the Mars Surveyor 2001 missic_n, the Robotic Arm will be used instead to deploy the Marie Curie Rover on the Martian surface (Figure 3). In this new approach, a 3-D terrain map generated by the Pancam Stereo Camera system will be used to determine the Rover deployment site. Two Rover deployment zones are defined. The primary Figure 2 Robotic Arm with Rover Model deployment zone is the area which is reachable by the Robot Arm and can be viewed by the Pancam. The Robotic Arm as a support instrument secondary deployment zone is the area which is reachable by the Robot Arm but cannot be viewed by the Pancam. If Support to the MECA - One of the primary mission goals is the Robotic Arm is forced to deploy the Rover in the to analyze soil samples in the MECA Wet Chemistry Lab. secondary zone, the non-stereo Robot Arm Camera (RAC) The Robotic Arm will support this goal by acquiring both mounted on the Robot Arm forearm will be used. surface and subsurface soil samples in its scoop from the area in the vicinity of the Lander and dumping the soil samples into the MECA wet chemistry cells and microscope port. Subsurface soil samples will be acquired at varying depths from within trenches excavated by the Arm, potentially to a depth of 50cm depending on the soil conditions. The Arm is capable of reaching deeper than 50cm below the surface, but operational constraints are expected to limit practical digging depth. The Arm will dump soil samples on the MECA material patch plates for imaging by the Robotic Arm Camera to measure properties such as soil particle wear, hardness, and adhesion. The Arm will also position the MECA electrometer for measuring triboelectric charge during digging and atmospheric ionization.

Support to the Robotic Arm Camera - A key element of the Mars Surveyor 2001 instrument suite is the Robotic Arm Camera (RAC) mounted on the forearm just behind the Figure 3 Robotic Arm Deploying Rover wrist. Soon after landing the Robotic Arm will position the RAC to take images of the Lander lbot pads, providing In picking up the Rover, a crowfix3t mechanism mounted useful data in determining surface properties at the on the Robot Arm wrist, together with a ball and wire touchdown site. Throughout the mission the Arm will mounted on the top surface of the Rover, will be used. This perkxtically position the RAC to take images of the surface, design allows +/-7 mm Robot Arm positioning error. In trench flcx)r and end walls, and dumped soil piles. During order to place the Rover on the Martian surface without bumpinginto the delicate Rover solar panel surface with shoulder, elbow, and wrist pitch. The Arm links are made the crowfoot, careful studies are necessary since Robot Arm of a low-mass graphite-epoxy composite. The end effector positioning, 3-D terrain map generation, and finding a consists of the following tools: a scoop for digging and soil stable positioning point for a given non-trivial terrain all sample acquisition, secondary blades for scraping, an have limited accuracy. In the visual approach, the Rover electrometer for measuring triboelectric charge and will be moved clown 3 cm (TBD) at a time, until the atmospheric ionization, and a crowfoot for deploying the crowfoot is disengaged from the ball. Other potential Rover. approaches that could reduce the total number of days for Rover deployment are motor current sensing, short-motor- The joint actuators consist of DC motors with 2-stage speed circuit, and open-motor-circuit approaches. These different reduction consisting of a planetary gearhead and harmonic approaches will be carefully investigated including drive (except the wrist, which has a bevel gear at the output thermal-vac tests, examining temperature dependencies. of the planetary gear). The actuators are capable of producing 26, 91, 53, and 10 Newton-meters of torque at Robotic Arm as a Science Instrument the joint output during normal operation for joints 1 through 4, respectively. Peak limits are approximately 50% During the surface operations of the Mars Surveyor 2001 higher. The amount of force that the Arm can exert at the payload, the Robotic Arm will also be used along with the end effector is configuration dependent, but is typically other Mars Surveyor 2001 instruments to investigate the around 80 N. Braking is achieved by actively shorting the physical properties of the surface and subsurface materials motor leads to slow the motor until magnetic detents in the workspace. The primary surface investigation by the Robotic Arm will be the direct measurements of the capture the rotor. Position sensing is accomplished via non- quadrature optical encoders at the motor shaft and mechanical properties using motor currents to estimate potentiometers at the joint output. Each joint is equipped Arm forces. Additional information will be obtained by with a heater (lW for the shoulder and elbow joints and judicious planning of Arm operations, such as purposeful 4W for the wrist joint) and temperature sensor to assure placement of excavated soil to observe the angle of repose that the motor operation is conducted at or above minimum and the degradation of the pile due to wind erosion. The temperature (208 K). See Table 1 for a summary of the Robotic Arm workspace activities will be tracked and Robotic Arm characteristics. mapped, and all pertinent Arm calibration and operations data will be archived for future investigations. The RA Electronics (RAE) consists of two PC boards which provides power conditioning; motor and heater drive Direct measurements by the MECA will provide additional circuitry; joint encoder counting; A/D conversion of information useful for understanding the physical potentiometer voltages, temperature sensor voltages, motor properties and chemical composition of the surface and subsurface materials. Much of the information about the currents, and total heater current. It also provides interface to the Lander Command and Data Handling (C&DH) soil will come from the RAC. The ability of the RAC to computer over a 9600 baud serial link. Firmware running provide close-up imaging of material on the tip of the scoop on the RAE microprocessor provides for low-level motor blade at 23 micron resolution is an example of how the data command execution to move the joints to the specified gathered by another instrument is highly dependent on positions, heater command execution, A/D calibration, and cooperative operation with the Robotic Arm - in this case to sensor monitoring. Digital data is updated at 2 ms deliver an appropriate sample to the RAC near focus intervals; analog data is updated at 20 ms intervals. viewing zone.

Software -The RA flight software resides on the Lander The majority of the physical properties experiments will be Command and Data Handling computer and provides the planned well in advance of landing. This is because following functions: previous in situ missions have left behind a strong history of materials properties investigations. In particular, the • Initialization (load parameter table and state files); Viking Lander mission investigations [4, 71 represent • Expansion of high-level task commands; appropriate approaches, which can easily be adapted for use • Generation of Arm movement trajectories; by the Mars Surveyor 2001 payload. Additional information • Control of Arm motion and joint heaters; provided by the unique capabilities of the Mars Surveyor • Setting parameters (e.g., motor current limits) in the 2001 payload will provide new insights in areas previously RAE. not possible. • Reading sensor data and monitoring the Arm status; • Fault detection and recovery; Robotic Arm Description • Sending Arm sensor data to telemetry. Hardware -The Mars Surveyor 2001 Robotic Arm is a 4- degree-of-freedom manipulator with a back-hoe design providing motion about shoulder yaw (azimuth) and Table 1 Robotic Arm Parameters

Parameter Value Comment

Degrees of freedom 4 rotary joints - shoulder yaw (azimuth), Back-hoe design. shoulder pitch, elbow pitch, wrist pitch.

Reach 2-m radius sphere

Max Cartesian velocity 0.07m/sec Configuration dependent.

Mass 5 Kg. Includes electronics (868g). Materials:

Upper Arm and forearm link Graphite-epoxy tubes. Scoop Blade 6AI-4V Ti STA Secondary Blades Tungsten Carbide, GC015

Actuators DC motors with 2-stage drive train Wrist has bevel gear for 2nd stage (planetary gear plus harmonic drive). instead of harmonic drive.

Accuracy and repeatability I cm and 0.5 cm, respectively.

End-effector force capability Configuration dependent; typically 80 Newtons.

Thermal environment:

Non-operating:

Shoulder, upper Arm, elbow 173 K to 308 K. Forearm, scoop, wrist 153 K to 308 K

Operating: Heaters used when necessary to bring Shoulder, upper Arm, elbow 193 K to 308 K actuator temperatures up to 208K before operation. Forearm, scoop, wrist 168 K to 308 K

Scoop volume TBD

Power 42W peak during heavy digging, 15W Load dependent. Values include 5W average during free space motion. for electronics.

Joint parameters See Table 2.

The Robotic Arm has a full suite of Arm motion commands period is 200 msec. during which the Arm state is that provide for coordinated joint motion as well as monitored for proper operation and the necessary control Cartesian motion of the selected tool [ 13]. Joint moves can inputs computed. A block diagram of the control system is be specified as either absolute moves or relative to the given in Figure 4. current position. Cartesian moves can be specified as absolute or relative moves with respect to the Mars The Arm can also be commanded to perform more Surveyor 2001 coordinate frame. The operator can also complicated tasks such as digging a trench or acquiring a specify Cartesian motion in the local frame of the currently sample by a single command. The software expands the selected tool (scoop blade, secondary blades, electrometer, high-level command into the appropriate set of motion RAC, and Mossbauer). The four degrees of freedom for commands which are executed sequentially. This not only Cartesian position are specified as the three translation saves uplink bandwidth, but eases the burden on the coordinates plus the angle that the currently selected tool operator in developing complicated command sequences. approach vector makes with the plane of the Lander deck. The software also tracks time and energy resources used during command execution and will gracefully terminate Each motion command is broken up into a series of via operations when allocations are exceeded. This feature points which are sent sequentially to the RAE for execution will be most useful when digging a due to the uncertainty by the firmware. The software control loop sampling Table 2. Robotic Arm Joint Parameters

Parameter Joint 1 Joint 2 Joint 3 Joint 4 Units

Actuator output torque 26 91 53 i0 Newton-meters

Gear ratios 12500 16O00 10000 40OO

Min angle -134.9 -138.0 -33.6 -20.2 degrees

Max angle 149.9 80.0 231.3 198.6 degrees

Max speed (no load) 2.0 1.5 2.5 6.1 deg/sec

Heaters I 1 1 4 watts

If a fault or event is detected, the fault or event type is reported in telemetry. Depending on the fault or event detected, the RA software will either attempt to recover from the fault or event or place the Arm in a safe configuration. It is expected that the Robotic Arm will occasionally encounter conditions that impede its motion during digging (a rock in the soil, a patch of ice, etc.). The software has a built-in accommodation algorithm, similar to the algorithm in [1], to compensate for this condition by adjusting the scoop trajectory and, if necessary, dumping the scoop contents and re-executing the digging motion.

Development Testing and Calibration

The Robotic Arm will be extensively tested during development to verify that the design can withstand the Figure 4 Robotic Arm Control System harsh environmental conditions expected as well as to characterize the performance of the actuators and to of the soil properties which affect the execution of the dig calibrate the sensors and kinematic model of the Arm. trench command. Qualification testing included both vibration to simulate launch loads and thermal-vacuum testing to simulate the In addition to providing for control of the free-space Arm Martian environment (temperature and pressure). motions, the software is also capable of executing guarded moves where the Arm will move towards its commanded The performance of the actuators will be evaluated over a position until contact is made. This is accomplished by temperature range of 183 K to 293 K and expected monitoring motor currents and computed joint torques operating voltages. Data from the characterization are used versus preset thresholds. Guarded moves will be employed by the control system to continuously monitor joint torques when positioning the Mossbauer on its targets, acquiring for use in executing guarded moves, in the accommodation samples, and when digging trenches. Thus, Robotic Arm algorithm and to prevent excessive torque from damaging operation is robust with respect to surface location the joints. The joint output torques are computed by first uncertainty. computing the no-load motor currents which are both temperature and voltage dependent and then computing the To aid in safety and increase autonomy, the Robotic Arm torque from the actuator torque constant. The no-load software is capable of detecting and recovering from faults motor currents are computed from and anomalous events. Faults and events are defined as follows: lnl = Io + ae-bT(V/Vmax)

* Fault - inability to complete a command due to failure and the joint torques from of hardware (sensor, actuator, electronics, etc.) or software; "c= Ka(i - Inl ) • Event - inability to complete a command due to where inl is the no-load motor current, !o is the no-load anything other than a fault (e.g., Arm motion impeded due to hard soil). motor current at 293 K, a and b are constants, T is the temperature, V is the applied motor voltage, Vnmx is the maximum operating voltage, x is the torque, / is the motor current,and Ka is the actuator torque constant. The moving the dump location for future digging to another constants a, b, and !o are determined from the test data by area. This means extra effort in managing the available using a least-squares fit. During the landed mission, a workspace as a resource, as well as the additional wear on standard set of free-space moves will be periodically the actuators for the additional movement, but the executed to monitor actuator performance. In addition, the supplementary data necessitates the effort. joint heaters will be operated to characterize the thermal properties of the joints in the Martian environment. In addition to the data gained during regular Arm operations, specific materials properties experiments will be Calibration of the Arm position sensors and kinematic performed (see Table 3). Because of the criticality of the model will be done moving the Arm through a series of efficient operation of the Arm to support the rest of the poses throughout the workspace and measuring the location science objectives (particularly acquiring samples for the of the end effector using a system of highly accurate MECA) , dedicated materials properties experiments will theodolites. The sensor and kinematic model parameters be done based on available resources. However, even under will then be determined by solving a constrained adverse conditions it should be possible to perform a minimization problem which minimizes the mean error substantial number of dedicated experiments. The over the measured poses. The kinematic model parameters following is a partial list of some of the physical properties are based on the method of Denavit and Hartenberg [2]. experiments that will be conducted: a) Scoop blade insertion to determine soil penetration During digging and soil-mechanics experiments, estimates resistance. of forces exerted by the end-effector tools are important b) Scraping with the scoop blade and the secondary blades. data for use in determining soil properties. These estimates The cutting ability of the different cutting tools will can be made from the sensed motor currents, but will be yield information on the cohesion of the soil. Close-up somewhat crude due to unmodeled Arm dynamics and the imaging of wear on the scoop blade will provide grain limited degrees of freedom of the Arm. During digging and strength data. If the opportunity is presented, rocks soil mechanics experiments, reaction from the soil can within the workspace will be abraded using the tools on exert forces on the end effector which cannot be detected at the scoop. the Arm joints via the sensed motor currents due to the c) Intentionally causing the trench to cave in. By under- limited degrees of freedom and the fact that all of the cutting the wall of the trench or by using the under side motors are not on at all times during Arm motion. End- of the scoop to apply pressure at the surface next to the effector forces can be estimated from edge of the wall it will be possible to cause a trench wall to cave in under controlled conditions, yielding bearing Fe = jT# strength data. d) Chopping soil samples. The ability of the Arm to where Fe is the force vector exerted by the end effector, repeatedly chop a sample in preparation for MECA jT# is the pseudoinverse of the manipulator Jacobian [10] delivery will provide cohesion data transpose with the rows associated with the unactuated e) Shaking the end effector. Because of the flexibility and joints removed, and "_is the vector of joint torques for the length of the Arm it is possible to create repeatable agitations to shake loose particles, allowing for insight actuated joints. End-effector forces in the null space of jT into particle adhesion. will not appear in the joint torques. The manipulator Excavated soil piles. Long term data will be gathered Jacobian is dependent on Arm configuration and, thus, the by monitoring the evolution of purposefully placed transformation to end-effector forces and the null space conical excavated soil piles. changes as the joint angles change.

The primary operations tool for commanding the Mars '01 Experimental Investigations Robot Arm will be the Web Interface for Telescience Data acquired as part of the physical properties experiments (WITS) system. WITS provides target designation from will come from many sources. A majority of the RA panorama image data, generates command subsequences operations will be in support of the primary mission via programmed macros, simulates arm motion, checks for objectives including: digging, dumping, acquiring samples. collisions, computes resources (energy, time, data), and Although these activities will not be performed specifically outputs a complete command sequence file for uplink to the to provide materials properties data, by tailoring the Lander. operational sequences carefully it will be possible to leverage this data with that from other instruments to gain Data Products additional insight. For example, by maintaining a constant The Robotic Arm subsystem generates two kinds of dump location for a few hours of operation while digging a telemetry - engineering and science. Engineering telemetry trench, a rather sizable conical pile can be obtained. In consists of current Arm state data which is downlinked at order for this pile to be useful for observing changes over the completion of each Robotic Arm command. Science time, it should be in an isolated area, which necessitates telemetry consists of detailed sensor data collected every Table3. Soil PropertiesExperiments

Property Task

Adhesion Imaging of scoop and patch plate.

Angle of internal friction Surface bearing tests using bottom of scoop, imaging footpads.

Angle of repose Imaging of natural slopes, trench walls, tailing piles.

Bearing Strength and Cohesion Imaging of footpads and trench wall.

Bulk Density Imaging of footpads.

Chemical Compositions MECA analysis.

Grain size distribution RAC imaging and MECA sorting on screen before and after vibration

Heterogenity RAC imaging, Arm forces while digging.

Penetration resistance Scoop blade insertion.

200 milliseconds during command execution. Robotic Arm with the reconstructed Arm trajectories will yield science telemetry is used for reconstruction of the digging information regarding the degree of difficulty of digging in process, soil-mechanics experiments and for trouble the various soils encountered and of executing the Arm shooting. motions during the various soil-mechanics experiments. In addition to the data listed above, detailed history of the The following engineering data is reported to the telemetry Arm state and control variables for the last one minute of system at the completion of each Robotic Arm command operation is downlinked whenever a fault or event occurs. (except where noted): This will permit reconstruction of the exact sequence of events leading to the anomaly. • Command op code; • Joint position from encoders (radians); The following data will be archived in the Planetary Data • Joint position from potentiometers (radians); System (http:pds.jpl.nasa.gov) for use by the science • Joint temperatures (degrees Celsius); community: • Sum of heater currents (amps, reported upon change); • Position data for the RAC; • Energy consumed (watt-hours); • Joint positions, temperatures and motor currents for • Voltage references (volts); reconstruction of Arm trajectories and joint torques; • Health status (reported upon fault or event) • Calibration report;

While the Arm is moving, raw Arm sensor data is collected • Experimenter's notebook. every 200 milliseconds and stored for subsequent downlink 3. ROVER in telemetry. All analog data is converted to 12-bit digital format. The following raw digitized data is collected: The Marie Curie rover (Figures 5 & 6) is a six-wheeled vehicle 68 cm long, 48 cm wide, and 28 cm high (with 17 • Joint angle encoder count; cm ground clearance). The body is built on the rocker- • Joint angle potentiometer voltage; bogie chassis, which, by use of passive pivot arms, allows • Joint temperatures; the vehicle to maintain an almost constant weight • Motor currents; distribution on each wheel on very irregular terrain. As a • Motor voltages; result, the rover is able to traverse obstacles about !.5 • Status word (motor, brakes, and heater state times as big as the wheels, since the rear wheels are able to information) maintain traction even while pushing the front wheels into • Sum of heater currents; vertical steps hard enough to get lifting traction. This • Time. consists of linkages, six motorized wheels, and four motorized-steering mechanisms. The four cornered steering mechanisms allow the rover to turn in place. The The Robotic Arm science telemetry will be the most useful vehicle's maximum speed is about 0.7 cm/sec. Since the for scientific analysis of soil properties during digging and design of the Marie Curie is very similar to that of the soil-mechanics experiments. The motor currents along Sojourner, more details of the design and implementation canbefoundin [51,[11], and [12]. If the rover ball and imaging. The rear color CCD camera is used for science wire cannot disengage from the Arm crowfoot during the imaging and APXS target verification. A suite of five rover deployment, a pin puller mechanism is mounted at infrared laser stripe projectors, coupled with the front CCD the center of the rover solar panel, and it can be released cameras, provide the proximity sensing and hazard by an operator command. detection capability for the vehicle. This system operates by locating the image of the laser stripes on a few selected scan lines of the camera images. Deviations of the detected locations from the nominal fiat-terrain values indicate that the terrain is uneven. An array of elevation values is created from the stripe-camera intercepts. Proximity hazards are detected when elevation differences between adjacent points in the array exceed a threshold, or when the difference between the highest and lowest point in the array exceeds a threshold. Other hazards include excessive roll or pitch, or excessive articulation of the chassis, or contact with bump sensors on the front or rear of the vehicle.

A bi-directional UHF radio modem (9600 bits/second) allows the vehicle to transmit telemetry and to receive commands from Earth via the Lander. The vehicle is powered by a 15-watt gas solar panel backed up in case of failure by a non-rechargeable Lithium battery. This battery Figure 5 The Marie Curie Rover is also used for nighttime APXS operations.

Rover Navigation

The rover is operated on the basis of a fixed local coordinate frame with origin at the center of the Lander

Rear_a Asse,_ly base and the X and Y axes pointed to Martian North and 7,== COlD_ East (right-hand rule), respectively (Martian North is defined by the Lander sun finder). The vehicle's X,Y Ro_w-8og_-O_mlrerllal $uz_ns_or_P_w'r_Uy positions are calculated (at -2 Hz rate) by integrating its odometer (average of the six wheel encoder counts) with _t_t_ Sor,_ _Uy the heading changes produced by the rate gyro. Due to the Fro_l Burr_ a_'ro low processor speed and lack of floating point arithmetic,

Wheel Dffve A,imembles Catnlmi._ 8,1 _ millimeter (mm) and Binary Angle Measurement (BAM)

smNiq _ _ ,it,wl_ltql i_,hi are used as distance and turn angle units respectively (1 Deg = 182 BAM or 360 Degs = 65,536 BAM). While moving, the vehicle monitors its inclination, articulation, Figure 6 The Rover Assembly contact sensing, motor and power currents, and temperatures to be sure they did not exceed limit conditions Electronics based on risk level settings. Being too close and heading toward Lander conditions are also monitored. The rover The rover is controlled by an lntel-8085 CPU operating at 2MHz (100KIPS). The on-board electronics are custom periodically sends a heartbeat signal to the Lander at one designed in order to meet the flight requirements and to fit vehicle-length intervals. In the absence of this into a small Warm Electronics Box (WEB). The on-board communication signal, the vehicle is autonomously backed up half of its length and a communication retry takes place. memory, addressable in 16 Kbyte pages, includes 16 Kbyte The rover motion is commanded by one of the following rad-hard PROM, 176 Kbyte EEPROM, 64 Kbyte rad-hard commands: Turn, Move, Go to Waypoint, Find Rock, and RAM and 512 Kbyte RAM. The navigation sensors Position APXS. consist of a rate gyro, 3 accelerometers for sensing the X, Y, and Z axis motion, and 6 wheel encoders tbr odometry. Articulation sensors include differential and left and right The Turn command in general causes the vehicle to change its heading in place. The four steered wheels are adjusted bogey potentiometers. Wheel steering and APXS (Alpha- into their appropriate positions, then the vehicle wheels are Proton X-Ray Spectrometer) positions are monitored by 5 turned until the desired heading, indicated by integrating potentiometers. All motor currents and the temperatures the rate gyro, is met. The rover completes a turn when the of vital components are also monitored. gyro heading is in within +/- 1.5 degrees of the desired The two front black and white CCD cameras (768 x 484 heading. In case the gyro is disabled, the odometry is used to calculate the heading changes; if both the gyro and pixels) provide hazard detection and science/operation odometeraredisabled, timing is used in the calculation. The Find Rock command is very similar to the Go to The Turn To command causes the vehicle to turn to a Waypoint command, except that after a hazard is detected specific heading, while the Turn By command causes the at approximately the X,Y position of the waypoint, then the vehicle to turn to a relative heading. The Turn At rover centers its heading between the edges of the rock command causes the vehicle to turn so as to point to a using proximity sensing. If the destination coordinates are specific X,Y position. reached without any rocks found along the way, a spiral search is performed until the rock is found. The Move command enables the vehicle to move for a specified distance, using only odometry and no hazard In both Go to Waypoint and Rock Finding commands, the avoidance. This "blind move" is useful when the terrain is rover reaches its destination when dX * dY < 100 ram2; dX clearly seen by the operator (in images from the Lander) and dY are distances from the vehicle to its target position and the move is a short one. The Set Steering Position in X and Y respectively. In case the rover can not get to its parameter of the Move command determines the arc radius destination due to an obstacle at the destination, the rover of the move. The rover completes a move once the average declares a successful command completion when it comes six- wheel encoder count exceeds the desired encoder within 500 mm z of the target destination. The vehicle count. Parts of the distance errors are due to the wheel monitors the progress of the Go to Waypoint and Find Rock slippage, and they depend on the terrain the vehicle commands and enforces a time limit (which is a parameter traverses. of the command).

The Go to Waypoint command causes the vehicle to The Position APXS command enables the vehicle to move traverse to a specified X,Y location. The vehicle drives backward until the APXS sensor head contacts the rock that forward a distance of one wheel radius and stops for laser has been found or until the maximum allowable distance proximity scanning. A terrain height map is constructed has been reached without contact or time-out. internally from the information provided by the lasers and CCD imagers. If an obstacle is detected on the left, the For every uplink command, the vehicle sends either an vehicle will turn right, and visa versa. A flag is set which acknowledge message or the telemetry collected during indicates the direction of the turn, and the vehicle will execution of the commands, including any error messages. continue turning by increments until a hazard-free zone at Navigation telemetry in general contains the time tag, the least as wide as the vehicle is detected by the laser scanning command sequence number, the current X,Y and heading system. If the clear zone is wider than the vehicle turning values, steering positions, inclination and articulation circle, then the rover drives straight ahead far enough to values, motor currents, temperatures, and contact and bring the obstacle alongside. Then the rover begins an arc encoder information. In addition, the Go to Waypoint and toward the goal point, clears all memory of the hazard Find Rock telemetry data also include the obstacle height avoidance maneuver, and continues. If the clear zone is map provided by the proximity and hazard avoidance narrower than the vehicle- turning circle (but wider than mechanism for every 6.5 cm of traverse. the vehicle) then a "thread-the-needle" maneuver is The health checks telemetry provides a snapshot of the attempted. This maneuver centers the rover on the current status of the vehicle. In addition to almost all of the perpendicular bisector between the two hazards, and moves navigation information, the power supply current and straight ahead along that line until a zone that is big voltage status, individual wheel odometer readings, enough to turn around is detected. Once such a zone is communication error counts, device fail counts, minimax detected, all memory of the maneuver is deleted and the accelerometer values, motor current values, and average rover begins an arc toward the goal. If an obstacle is motor currents of the last traversal are reported here. Other encountered prior to detection of a free turning circle, then rover telemetry data is designed to report data from science, the rover backs straight out to the point where the thread- engineering experiments and rover housekeeping utilities. the-needle maneuver began, and the rover continues to turn until another hazard-free zone is detected. Arcs toward the The rover also has the ability to adjust its position goal are calculated to three values: if the rover is already knowledge based on the assessment of the Lander using the pointed toward the goal (within a small deadband) then the Lander Based Autonomous Localization (LBAL) algorithm. rover goes straight, if the rover heading is outside that For every heartbeat which the rover sends to the Lander deadband but less than about I radian, then a large-radius during Go to Waypoint or Find Rock command execution, turn (about 2 meters) is begun which turns toward the goal, the Lander's response will be based on whether the LBAL and if the heading is more than 1 radian from the goal function is active or not. If the Lander's LBAL function is direction, then a short radius turn (about 1 meter) is begun not active, the rover will continue on with its navigation which turns toward the goal. Note that a turn in place task. Otherwise, the Lander will request the rover to wait maneuver is not used here, since that would cause the rover for the Lander to send to the rover the updated position to become trapped in "box canyons" whereas the present information. The Lander uses the rover position algorithm does not. information from the heartbeat message to capture a stereo pair of images with its cameras pointing toward the general area where the rover has stopped. The Lander's on-board LBAL algorithm will estimate the current rover position The sensor head is mounted to the APXS Deployment based on these images, and will send this new rover Mechanism (ADM). Through a command from the position information to the rover for updating. At any time operator, the ADM enables the APXS sensor head to be in between commands, the operator can also request the placed closer to but not contacting the experimental rock or rover to update its position by sending a LBAL request soil. The APXS electronic unit serves as an interface command to the rover. Subsequently, the rover will perform between the APXS sensor head and the rover on-board a heartbeat session to get its new position from the Lander. computer system. The electronic unit receives commands from the rover computer and sends appropriate signals to The Rover Control Station the sensor head. The electronic unit also accumulates APXS sensor head data before sending back to the rover Human operators using the Rover Control Workstation computer. The PIs of this instrument are from the Max- (RCW) indirectly control the rover. The RCW's Planck-Institute fur Chemic, Mainz, Germany and from the customized graphical user interface software provides tools Enrico Fermi Institute, University of Chicago, Chicago. for the operator to generate commands with parameter checking capabilities and to designate waypoints in a 3-D The Material Experiment on Electrostatic Charging image display. A command sequence which comprises (MEEC) experiments consists of two components: the multiple commands is built based on requests from the Wheel Abrasion Experiment (WAE) and the measurement scientists, vehicle engineering telemetry, and the end-of-sol of the charges during a rover traversal by means of the stereo images captured by the Lander cameras. The rover potential the rover attains during the movement. 3-D icon shown on the RCW display allows the operator to assess traverse ability by placing the icon over a 3-D The first MEEC component, the WAE instrument [3], Martian terrain image set at any position and orientation. Figure 7, is designed to measure how much adhesive and The rover's current position and heading are also acquired abrasive the Martian dust would be on strips of pure metal by matching the icon with the rover's physical position in attached to one of the rover wheels. Fifteen thin film the stereo images. This capability allows the operator to re- samples (five each of three different metals) which are initialize the vehicle's true position and orientation at the attached to the wheel periphery reflect sunlight to a beginning of a sol. In Go to Waypoint designation, the photovoltaic sensor. The wheel rotation enables the operator specifies the rover destinations by placing the presentation of the different sample surfaces to the sensor. rover 3-D cursor at each waypoint, then clicking the mouse The resulting signals are recorded using the rover computer to identify these destinations. The RCW records these and are interpreted in terms of dust adhesion and abrasive waypoints and generates the Go to Waypoint commands wear. automatically. Other commands are generated from operator-specified parameter values, and the command sequence file is created. The accuracy of the designation depends on the distance between the stereo cameras, image resolution, and human designation ability. The overall accuracy of the designation was estimated at about 2 to 3 percent for cross and down ranges and for heading.

The Science Instruments

There will be three science instruments mounted on-board the rover: The Alpha Proton X-ray Spectrometer (APXS), the Mars Experiment on Electrostatic Charging (MEEC), and the Wheel Abrasion Experiment (WAE) instruments. JI'L--: .i.L"3,.'

Figure 7 Reflective wheel (left) for WAE instrument The APXS instrument [81 is designed to obtain the chemical composition of Martian rocks and soil. The APXS uses three kinds of interactions of the alpha particles from The second MEEC component consists of a ground a radioactive source with matter: Rutherford backscattering reference watchplate nlounted at the front left corner of the (alpha mode), nuclear reactions of alpha with some light rover solar panel, and an electronic board for collecting data. A 100-microCurie Americium 241 dot resides on the elements (proton mode), and generation of characteristic x- ray in the sample through ionization by alpha particles (x- watchplate and will be used to ionize some of the Martian ray mode). Three energy spectra obtained from these atmosphere to "ground" itself to Mars. The potential the interactions are recorded in three different channels. In the Marie Curie rover attains during a traversal (or overnight alpha mode, the APXS can measure all chemical elements, or Martian wind) with respect to the "ground" reterence which are heavier than helium. Sensitivity is excellent for will be measured by the electronic I:x)ard residing inside the light elements such as C, H and O. In the x-ray mode, it WEB. The potential difference between this dot and the measures all elements which are heavier than Na. The Rover chassis will be measured in terms of nanoAmp instrument includes a sensor head and an electronic box. current, which flows when the two are connected. This current will be time integrated to acquire a value of Although the Marie Curie rover is almost identical to the electrical charge. This value will then be used together with Sojourner rover, which landed on Mars in 1997, many the measured rover capacitance to estimate the rover improvements have been made to prepare for the upcoming electrical potential relative to the charge cloud. The mission. The rover mechanical and electronics hardware collected data will be convened to a digital format for have been refined. The rover's gyro and accelerometer transmission back to earth for study. The PIs of the MEEC performances have been iniproved significantly in terms of are from the NASA Glen Research Center, Cleveland, accuracy and reliability. The rover software has been Ohio. modified to cope with the new requirements in the areas of communication, landing and deployment profile, LBAL, 4. CONCLUSION pin puller and MEEC instrument. Some time in January of 2002, the Marie Curie rover will visit a whole new area on The Mars Surveyor 2001 Robotic Arm is an essential Mars in the vicinity of the Lander's landing site. The rover element in carrying out the Mars Surveyor 2001 science has the abilities of traversing to designated sites, of experiments. In support of the other instruments, it will examining soil and rocks using its on-board science dig trenches in the Martian soil, deliver soil samples to the instruments, and of capturing images of interest sites at MECA, and position the Mossbauer and the RAC. The close range or from distance. These sites could be far away Robotic Arm will also conduct Arm-specific science from the contaminants, which may have been caused by the experiments to collect data relating to soil properties such Lander propulsion system at the landing site. The collective as periodic imaging of dumped soil piles, surface scraping data from the rover will be analyzed and these will bring us and soil chopping experiments, compaction tests, insertion up-to-date knowledge at another part of the Martian surface of the scoop blade into the soil, scoop shake tests and which we have never visited before. trench cave-in tests. Key data elements include joint motor currents and trajectories which will be used to estimate end- ACKNOWLEDGEMENT effector forces during Arm operations. Data from the Robotic Arm support operations and science experiments The research described in this paper was carried out by the when combined with data from the other instruments will Jet Propulsion Laboratory, California Institute of yield important information on Martian soil properties, Technology, under a contract with the National providing valuable insight into the history of Mars. Aeronautics and Space Administration.

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Bonitz is currently with the Nguyen is a software engineer for Telerobotics Research and Applications Group at the Jet the Robotic Vehicles Group at the Jet Propulsion Propulsion Laboratory where he recently designed and Laboratory, Pasadena, California. Currently, he is the developed the control algorithms and software for the Mars software lead for the Microrover of the 2001 Mars Volatiles and Climate Surveyor Robotic Arm inherited for use Surveyor Project. Prior to this, he was a Microrover's by the Mars 2001 Lander Robotic Arm. Previously, he has software team member and a Microrover Downlink Data conducted research in control algorithms for multiple- Analysis team member of the Mars Pathfinder Project. manipulator robotic systems, robust internal force-based Since 1988, he had developed software and integrated impedance controllers, frameworks for general force hardware in the areas of motion control, and navigation decomposition, optimal force control algorithms, and for several planetary rover research programs. Prior to calibration methods for multi-arm robotic systems. He has these programs, he was a software team member of the worked for a variety of industrial companies including Telerobotics Research Project, and of the Robotics Raytheon, TRW, Source 2 International, and GTE. He has a Technology Test Vehicle Research Project. He was a key PhD in Electrical Engineering from the University of software engineer for the Three Axis Acoustic Levitator California, Davis. Project, a physics experiment flown on-board the Shuttle. Nguyen holds a BS in Physics and Chemistry. and a MSEE from CSU Long Beach, California.

| Won S. Kim received the Ph.D. degree in electrical engineering and computer sciences from the University of California, Berkeley, in 1986. He has been with the Jet Propulsion Laboratory, Pasadena, CA, since 1988. He received Franklin V. Taylor Award in 1988 IEEE Conference on System, Man, and Cybernetics, and published more than 50journal and conference papers in the telerobotics area.